Surface roughness of composite filling materials.

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Surface Roughness of Composite Filling Materials

HENRY L. LEE, Ph.D., JAN A. ORLOWSKI, Ph.D., and PATRICKD. KIDD, B.S. Lee Pharmaceuticals South E l Monte, California 91733

ABSTRACT Seven commercial composite filling materials were studied

as regards 1) differences in their ability to take a smooth polish as measured by an electronic roughness gauge and 2) their relative loss in smoothness due to toothbrushing. A s e r i e s of test formulations indicated that polishability and ability to retain a good polish are interrelated and are functions of filler particle size and filler hardness. Optimization in the filler particles from the point of view of polishability also results in improved wear resistance to toothbrushing.

I.

INTRODUCTION

The ability to take a good polish and to maintain the polish during service a r e important clinical properties of dental filling materials. Rough or irregular surfaces of dental fillings a r e not only uncomfortable and irritating, but can also increase the formation and adhesion of dental plaque and make i t s removal more 503 Copyright 0 1976 by Marcel Dekker, Inc. All Rights Reserved. Neither this work nm any part may be reproduced or transmitted in any form o r by any means, electronic or mechanical, including photocopying, microfilming, and recording, o r by any information storage and retrieval system, without permission in writing from the publisher.


504

LEE, ORLOWSKI, AND KIDD

difficult [l, 21. Attempts to provide the required smoothness in composite- type filling materials have shown that significant differences in the surface finish a r e obtained with different finishing techniques o r with different brands of composite materials [3, 41. Degradation of the surface finish by toothbrushing also has been shown for some composite filling materials [5]. The production and retention of a smooth surface on a composite filling material may depend to a great extent on physical properties and formulation variables of the material such as hardness, filler particle size, particle hardness, shape and crystalline form of the filler, filler concentration, and strength of the adhesive bond between individual filler particles and the resin matrix. Most of these factors have been shown to play a role in the wear resistance of composite filling materials to both toothbrushing and impact forces [6, 71, thus presenting the strong possibility that erosion and surface roughness development a r e closely linked to causal agents and processes. Since qualitative differences have been found in polishability among different brands of composite filling materials, it was deemed worthwhile to evaluate quantitatively the surface roughness of several commercial composite filling materials and some specially formulated test composite materials, both after finishing and after simulated toothbrushing, with a view toward determining the factors which influence the achievement and retention of a smooth surface. The surface roughness values of amalgam restorative material and tooth enamel were determined so that a rational basis for judging performance of the composite filling materials would exist. Finally, a new polishing paste, designed for use on composite filling materials, was compared in effectiveness to other polishing materials, The materials, techniques, and instruments employed in this study a r e accessible to all laboratories concerned with materials science. Results may be determined quantitatively, objectively, and reproducibly. It is only in this way that real improvements can be made in clinical applications and human health. 11. M A T E F t I A L S AND T E S T E Q U I P M E N T

Seven commercial composite filling materials were tested (Table 1). All were of the two-paste type. One material, Composite E, is intended to be a coating material and not a filling material, and it contains microfibers rather than the crystalline o r amorphous filler particles found in the other composites. Also tested were a polymeric glazing material G, an amalgam H, and extracted human teeth.

SURFACE ROUGHNESS OF COMPOSITE FILLING MATERIALS

505

TABLE 1. Composite Filling Materials Tested ~~

~


A Adaptic, Johnson and Johnson Co., New Brunswick, New J e r s e y 08903 B

Concise, Minnesota Mining and Manufacturing Co., St. Paul, Minnesota 55101

C

Cosmic, DeTrey, Zurich, Switzerland

D

Exact, S. S. White, Philadelphia, Pennsylvania 19102

E

Enamelite, Lee Pharmaceuticals, S. El Monte, California 91733

F

Prestige, Lee Pharmaceuticals, S. El Monte, California 91733

G

Finite, Lee Pharmaceuticals, S. El Monte, California 91733

H

Spheraloy, K e r r Mfg. Co., Detroit, Michigan 48174

Some important properties of the materials tested are given in Table 2. The average particle sizes listed in Table 2 were determined using a Micromeritics Sedigraph 5000 (Micromeritics Instrument Corp., Norcross, Georgia). Four test formulas of composite filling materials were prepared to study the effect of filler particle size alone on surface characteristics. The formulations of the materials a r e given in Table 3. Average particle sizes were determined using the Sedigraph 5000. The composite filling materials studied were mixed in equal parts by weight per the manufacturers' directions and cured for 10 min a t 37°C in a circular sample mold made of polished stainless steel with a diame t e r of 25 mm and a height of 5 mm. The top and bottom of the mold was formed by Mylar strips held by glass plates, so that all of the samples were initially cured against the Mylar strip. One-third of the cured samples were finished using a Shofu composite point (Shofu Dental Co., Menlo Park, California) until the smoothest (as measured) possible surface was obtained, and one-third were finished using a prophy cone and Precise Polishing Paste (Lee Pharmaceuticals Co., S. E l Monte, California), again until the smoothest surface was obtained. The polishing was done by dentists on coded samples. The remaining samples were allowed to cure an additional 24 hr in water a t 37°C and then subjected to simulated toothbrushing. The glazing material G was a liquid-paste system which was mixed one part liquid to four parts paste by weight. Samples were cured and finished in the same manner as the composite filling materials. Amalgam H samples were prepared with a 52% alloy and 48% mercury. Sample chips were prepared by hand-compressing the amalgam into the

5.5 7.0

Glass

Quartz Calcium silicate Silicabarium glass

103

103

106

96

113

100

95- 110

C D

E

F

G

H

11-0.10

-

-

20-0.35

22-0.40

25-0.25

32-0.20

36-0.30

6.5

6.5

4.5

7.0

74- 1.2

(wn)

-_

Size range

-

1.1

3.6

6.4

6.5

7.5

7.7

12.0

-

~~~-

Average filler sizea (!Jm)

aEquivalent spherical diameter. One half of mass consists of larger filler particles and half is in smaller particles.

(Amalgam)

Silica

Quartz

7.0

B

Quartz

105

Filler hardness (mohs)

A

Filler composition

Hardness, Rockwell H

Material

TABLE 2. Properties of Fillers Used in Some Commercial Composite Filling Materials


9j 0

r

V

SURFACE ROUGHNESS O F COMPOSITE FILLING MATERIALS

507

TABLE 3. Composition of T e s t Composites

1

Material Artif Cells Blood Substit Immobil Biotechnol Downloaded from informahealthcare.com by Queen's University on 12/30/14 For personal use only.

Binder

2

3

4

-60% diglycidyl ether of bis-phenol A dimethacrylatea -40% tetraethylene glycol dimethacrylate3% of benzoyl peroxide

*

Filler

4

silica

*

Fil1er:re sin wt ratio

4

2.2:l

*

Catalyst

Average filler particle size, pm 37

21

7.8

3.0

25 mm X 5 mm molds which had f i r s t been filled to a depth of 3.5 mm with a cured composite. The surface was smoothed with a hand scaler and allowed to set a t 37°C for 72 hr before polishing with pumice powder, The human teeth were two upper central incisors and a third molar. One of the incisors was subjected to brushing in the same manner as the composite samples and amalgam. The surface roughness of the teeth was measured on the lingual surface of the enamel. 111.

A.

METHODS

M e a s u r e m e n t of S u r f a c e R o u g h n e s s

The surface roughness of a l l test samples was determined with a Mitutoyo Surftest B roughness analyzer (Mitutoyo Mfg. Co., Tokyo, Japan) and a Brinkman s t r i p chart recorder, model 2543 (Brinkman Co., Long Island, New York), shown in Fig. 1. The combination produced both a surface profile graph and a numerical value of surface roughness, known as the center line average (CLA) value. This value is the average absolute deviation of the profile curve from the arithmetic mean within the reference length L:

CLA= -

0 (x)dx dx



508

FIG. 1. View of surface roughness tester.

L CLA

1

b

X

FIG. 2. Diagram illustrating the definition of center line average values. where 6 (x) is the actual surface profile. T h i s is illustrated graphically in Fig. 2. The CLA value depends on the reference length value, o r cut-off value, since the contributions to roughness by irregularities with peak-to-trough distances much g r e a t e r than L will be averaged to zero. In this study the cut-off value was 76 pm since it is mostly the smaller irregularities which contribute to the rough feel of a composite filling material. Since CLA v a r i e s from point to point in any


509


sample, the numerical read-out always consisted of a range of values. The midpoint of the range was taken to be the CLA for any one sample. The value listed for any material is the average of three samples. B. T o o t h b r u s h i n a S i m u l a t i o n

The test materials were brushed in a toothbrushing simulator previously described by Lee e t al. [7]. All samples were brushed a t a rate of 360 strokes/min with an Oral B, Adult 60 brush (Oral B Co., Bedford Hills, New York), and a s l u r r y of Crest dentifrice (Proctor and Gamble, Cincinnati, Ohio) under a constant load of 300 g. The samples were rotated to prevent channeling and to distribute wear evenly. Following brushing, samples were cleaned ultrasonically and dried a t 40°C to constant weight. Wear losses were determined as weight losses and were converted to micrometers of material lost per hour. All tests were run in triplicate.

IV. R E S U L T S Values of CLA roughness for commercial test materials a t a cut-off distance of 76 m, finished by three methods and after simulated toothbrushing, a r e given in Table 4. The wear r a t e s of each material by simulated brushing are also listed. For all of the composite filling materials, the smoothest finish was produced by curing against a Mylar strip. Finishing with Shofu points was superior to Precise paste for materials A, B, and D, whereas the paste was better for materials C, E, F, and G. A comparison of the actual surface profiles of the composite materials finished by both Shofu and Precise is shown in Fig. 3. Toothbrushing roughened the surfaces of all the composite materials, Actual surface profiles of the brushed materials a r e presented in Fig. 4. Brushing of material A produced the poorest surface with a CLA value of 4.25 i 0.05 pm. Surface roughness of the extracted teeth varied from 0.35 to 0.42 pm. Brushing of one tooth increased roughness from 0.39 to 0.65 pn. Materials C, E, and F retained surfaces comparable to the brushed tooth enamel. Amalgam H retained a very smooth surface during brushing, 0.10 i 0.02 pm. The glaze G retained the smoothest surface of any of the materials tested, 0.07 f 0.01 pm.


510


TABLE 4. CLA Values and Wear Rates for Dental Materials

Wear by brushing (pm/hr) after 86,400 strokes

Material

Finish technique

CLA (pm)

A

Mylar s t r i p Shofu point Precise paste Mylar s t r i p

0.39 1.50 1.75 4.25

k0.04 kO.20 iO.25 k0.05a

5.6

Mylar s t r i p Shofu point P r e c i s e paste Mylar s t r i p

0.34 1.15 1.55 2.40

kO.07 k0.15 kO.20 k0.20a

5.1


0.09 k0.01 0.61 i 0.04 0.41 kO.07 0.75 i O . l O a

2.3


0.25 0.72 0.85 1.85

kO.05 kO.08 iO.05 i0.05a

3.1


0.043 0.41 0.17 0.41

0.005 kO.02 k0.01 *0.06a

8.2


0.085 0.80 0.45 0.62

f 0.015

G

Mylar s t r i p Shofu point Precise paste Mylar s t r i p

0.045 k 0.005 0.61 i 0.03 0.17 iO.01 0.07 k O . O l a

7.9

H

Pumice

0.10 k 0.02"

11.5

Tooth structure

Tooth Tooth Tooth Tooth

1 2 3 3

aValues after brushing.

k

kO.11 kO.05 i0.03a

0.35 0.42 0.39 0.65a

1.6

SURFACE ROUGHNESS O F COMPOSITE FILLING MATERIALS WLISHEO WITH SHOFU COMPOSITE W I N 1

511

HE0 WITH PRECKE

A

I


IOU

11or

E

A-

t-

lcm

1

I

f

lcm

110”

-1

FIG. 3. Comparison of surface profiles of various composite filling materials after polishing by two techniques. The brushing wear rate data given in the last column of Table 4 indicate that amalgam was eroded most rapidly, while composite F had the lowest rate of wear.


512 LEE, ORLOWSKI, AND KIDD

D

F 110.

I’10”

FIG. 4. Comparison of surface profiles of various composite filling materials after toothbrushing.


513


V . DISCUSSION The results for the filling materials indicate that significant differences exist in the polishability and retention of surface smoothness after brushing between different brands of composite filling materials as well as between different classes of dental materials. For all composite materials the maximum initial surface smoothness w a s obtained when the materials were cured against a Mylar strip; however, in actual clinical practice it is more often the case that additional shaping and finishing operations, which negate the Mylar effect, a r e required on placements of composite filling materials. Under these conditions it w a s seen that the optimum finishing technique depends on the brand of composite material. In general, the use of a polishing stone i s preferred for finishing the quartz-filled materials such as A, B, and D, whereas the use of a composite-polishing paste is preferred for the glass-, silica-, o r silicate-filled materials such as C, E, F, and G. These results suggest that composite materials with particularly hard filler particles require harder abrasives for the smoothest finish. It should be noted that filler particle hardness i s not absolutely correlated with composite hardness; this i s probably due to differences in formulation, resin, and formulating techniques. The results of surface roughness tests on tooth enamel suggest that CLA values on the order of 0.6 pm a r e required for a smooth-feeling composite. While several of the composite materials tested (C, E, F, G) retained the required smoothness after brushing, even the best of the quartz-filled composite materials had a CLA value twice the enamel value. The excellent surface characteristics of amalgam were evident with a CLA value after brushing of 0.10 pm, but one composite material, G, retained a smoother surface of 0.07 pm. This material is, however, a glaze coating for older, rougher quartz-filled composite restorations, rather than a filling material. Figure 5 shows the effect of varying the average filler particle size on the surface roughness of the special composite formulations. The standard deviations are indicated by the vertical lines. Surface roughn e s s obeys a linear log-log relationship when plotted against the average particle size. The lines obtained for the data before and after brushing a r e parallel, indicating that the roughness increases in the same ratio independently of filler particle size. The effect of filler hardness on surface roughness, both after finishing and after brushing, was evaluated by considering the data only for the commercial composite filling materials with an average filler particle size between 6 and 8 pm, as given in Table 2. In this narrow range the effect of particle size on roughness would not be



5 14

1

2

I

I

5

10

20

50

AVERAGE PARTICLE SIZE, pm

FIG. 5. Dependence of surface roughness of dental composites on filler particle size: ( 0 ) after finishing by b e s t method and ( m ) after brushing. too important. The results a r e shown in Fig. 6, in which surface roughness is plotted against the Moh hardness of the filler particles. Both polishability and retention of surface smoothness after brushing deteriorate with increasing filler hardness. The results in Fig. 5, showing the increase in surface roughness with increase in filler particle size for the special test composites, suggest that when other factors a r e held constant, the size of filler particles has an important effect on both polishability and smooth-


515


2.5-

2.0-

1.5-

1 .o-

0.5-

I

0.0

4

5

6

7

0

FILLER HARDNESS, moh

FIG. 6. Dependence of surface roughness of commercial composites on filler hardness for average filler size between 6 and 8 l m : ( 0 )after finishing by best method and (a) after brushing. surface retention of a composite filling material. In order to determine the effect of particle size on the surface roughness of the commercial composite materials, only the data were considered for the materials with fillers of hardness of 6.5 to 7.0 mohs, so as to minimize the effects of filler hardness. The data are plotted in Fig. 7, which shows the effects of average filler particle size on surface roughness before and after brushing. A similar relationship



5 16

2

5

I

I

I

10

20

50

AVERAGE PARTICLE SIZE, Mrn

FIG. 7. Dependence of surface roughness of commercial composites on filler particle size for fillers of hardness greater than 6.5 moh: ( 0 )after finishing by best method and (m) after brushing. between filler size and polishability exists for the commercial materials as for the special t e s t formulations. The apparently greater dependence of the CLA on particle size in the case of the commercial materials is probably due to their higher filler loading compared to the test composite formulations. Differences in composition of the resin component also may be one of the more important factors influencing the surface condition



517

of the materials. In addition, it is known that the use of coupling agents and other surface preparation techniques results in increased strength of the filler-resin bond in the composites. Increasing the strength of the filler-resin bond would seem likely to reduce the incidence of filler particles being torn from the composite surface to leave sizable holes in the surface during finishing operations or during brushing. The relationship between surface roughness and erosion by wear during brushing is shown in Fig. 8. Only the data for the commercial composite materials with fillers harder than 5.0 moh and of average size greater than 3 pm have been considered. The results support the proposition that factors which improve the surface smoothness of composite filling materials with f i l l e r s in the above ranges of size and hardness can also reduce the erosion effects due to wear. The specialized composites E and G, not included on the graph, appear to wear by a different mechanism. That there may be several mechanisms involved in wear resistance is indicated by the results of in vivo qualitative tests [8, 91, a modified abrasion test [lo], and an impact-abrasion test [?I.

0.0 0.0

1

1

2.0

4.0

CENTER LINE AVERAGE, pm

FIG. 8. Relationship between wear rate by toothbrushing and surface roughness after toothbrushing of commercial composite filling materials.



518

It appears that the use of smaller and somewhat softer filler particles in composite filling materials can lead to substantially improved surface characteristics. This has been achieved f o r seve r a l commercial materials. Changes in composite formulation variables considered in this study, such as p a r t i d e size distribution, o r other factors, such as the use of surface lubricating agents, may lead to further improvements in both surface smoothness and wear resistance and deserve further study. VI.

SUMMARY

Studies were conducted to determine some of the factors important to the polishability and retention of surface smoothness after brushing of restorative materials. Six commercial composite filling materials, four specially prepared composites, one composite glaze material, and three extracted teeth were used. Surface roughness was measured after finishing and after brushing. Finishes were obtained by curing against a Mylar strip, with a polishing stone, and with a polishing paste. The methodology of preparation and testing was given in detail so as to enable other laboratories to confirm our findings. The optimum finishing technique in c a s e s where cure against a Mylar s t r i p does not yield the c o r r e c t contour depends on the particular brand of composite material, with the use of a special polishing paste indicated for nonquartz filled composite materials. In the ranges tested, the surface roughness, both after finishing and a f t e r brushing, was dependent on filler size and filler hardness. Materials with relatively large- sized quartz particles showed the greatest roughness. Wear by toothbrushing could be correlated with the surface roughness after brushing.

REFERENCES [l] R. H. Roydhouse, Materials in Dentistry. A Discussion for Users of Dental Materials, Year Book Medical Publishers, Chicago, 1962. [2] E. W. Skinner and R. W. Phillips, The Science of Dental Materials, Saunders, Philadelphia, 1967. [3] P. 0. Glantz and L. A. Larson, "Surface Roughness of Composite Resins before and after Finishing," Acta Odontol. Scand., 30, 335-347 (1972). ~[4] J. B. Dennison and R. G. Craig, "Physical Properties and



519

Finished Surface Texture of Composite Restorative Resins," J. Amer. Dent, ASSOC., 85,101-108 (1972). R. B. Ward and W. B. Eames, "Surface of Filled Resins Produced b y Several Finishing Techniques," P a p e r given a t the 49th General Meeting of the International Association for Dental Research, 1971. R. L. Bowen, "Effect of Particle Shape and Size Distribution in a Reinforced Polymer," J. Amer. Dent. ASSOC., 69, 481-495 (1964). H. L. Lee, J. A. Orlowski, P. D. Kidd, W. R. Glace, and E. M. Enabe, "Evaluation of Wear Resistance of Dental Restorative Materials," in Advances in Polymer Friction and Wear (L. Lee, ed.), Plenum, New York, 1974, pp. 705-723. R. W. Phillips, D. R. Avery, R. Mehra, M. L. Swartz, and R. J. McCune, "Observations on a Composite Resin for Class I1 Restorations: Three-Year Report," J. Prosthet. Dent., 30, 891-897 (1973). W. B. Eames, J. D. Strain, R. T. Weitman, and A. K. Williams, "Clinical Comparison of Composite, Amalgam, and Silicate Restorations," J. h e r . Dent. ASSOC., 89, 1111-1117 (1974). J. M. Powers, L. J. Allen, and R. G. Craig, "Two-Body Abrasion of Commercial and Experimental Restorative and Coating Resins Ibid., -89, 1118-1122 (1974). and an Amalgam," Received by editor February 6, 1975

Letter: 'Composite filling materials as adhesives in orthodontics'.

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